Laser processing

ABSTRACT

The invention provides a system and method for vaporizing a target structure on a substrate. According to the invention, a calculation is performed, as a function of wavelength, of an incident beam energy necessary to deposit unit energy in the target structure. Then, for the incident beam energy, the energy expected to be deposited in the substrate as a function of wavelength is calculated. A wavelength is identified that corresponds to a relatively low value of the energy expected to be deposited in the substrate, the low value being substantially less than a value of the energy expected to be deposited in the substrate at a higher wavelength. A laser system is provided configured to produce a laser output at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate. The laser output is directed at the target structure on the substrate at the wavelength corresponding to the relatively low value of the energy expected to be deposited in the substrate, in order to vaporize the target structure.

BACKGROUND OF THE INVENTION

This invention relates to laser processing systems and methods,including systems and methods for removing, with high yield, closely

spaced metal link structures or “fuses” on a silicon substrate of anintegrated circuit or memory device.

Laser systems can be employed to remove fuse structures (“blow links”)in integrated circuits and memory devices such as ASICs, DRAMs, andSRAMs, for purposes such as removing defective elements and replacingthem with redundant elements provided for this purpose (“redundantmemory repair”), or programming of logic devices. Link processing lasersystems include the M320 and M325 systems manufactured by GeneralScanning, Inc, which produce laser outputs over a variety ofwavelengths, including 1.047 μm, 1.064 μm, and 1.32 μm.

Economic imperatives have led to the development of smaller, morecomplex, higher

density semiconductor structures. These smaller structures can have theadvantage of operation at relatively high speed. Also, because thesemiconductor device part can be smaller, a greater number of parts canbe included in a single wafer. Because the cost of processing a singlewafer in a semiconductor fabrication plant can be almost independent ofthe number of parts on the wafer, the greater number of parts per wafercan translate into lower cost per part.

In the 1980s, semiconductor device parts often included polysilicon orsilicide interconnects. Although poly-based interconnects are relativelypoor conductors, they were easily fabricated using processes availableat the time, and were well

suited to the wavelengths generated by the Nd:YAG lasers commonlyavailable at the time. As geometries shrank, however, the poorconductivity of polysilicon interconnects and link structures becameproblematic, and some semiconductor manufacturers switched to aluminum.It was found that certain conventional lasers did not cut the aluminumlinks as well as they had cut polysilicon links, and in particular thatdamage to the silicon substrate could occur. This situation could beexplained by the fact that the reflection in aluminum is very high andthe absorption is low. Therefore, increased energy must be used toovercome this low absorption. The higher energy can tend to damage thesubstrate when too much energy is used.

Sun et al., U.S. Pat, No. 5,265,114 advances an “absorption contrast”model for selecting an appropriate laser wavelength to cut aluminum andother metals such as nickel, tungsten, and platinum. In particular, thispatent describes selecting a wavelength range in which silicon is almosttransparent and in which the optical absorption behavior of the metallink material is sufficient for the link to be processed. The patentstates that the 1.2 to 2.0 μm wavelength range provides a highabsorption contrast between a silicon substrate and high

conductivity link structures, as compared with laser wavelengths of1.064 μm and 0.532 μm.

SUMMARY OF THE INVENTION

The invention provides a system and method for vaporizing a targetstructure on a substrate. According to the invention, a calculation isperformed, as a function of wavelength, of an incident beam energynecessary to deposit unit energy in the target structure. Then, for theincident beam energy, the energy expected to be deposited in thesubstrate as a function of wavelength is calculated. A wavelength isidentified that corresponds to a relatively low value of the energyexpected to be deposited in the substrate, the low value beingsubstantially less than a value of the energy expected to be depositedin the substrate at a higher wavelength. A laser system is providedconfigured to produce a laser output at the wavelength corresponding tothe relatively low value of the energy expected to be deposited in thesubstrate. The laser output is directed at the target structure on thesubstrate at the wavelength corresponding to the relatively low value ofthe energy expected to be deposited in the substrate, in order tovaporize the target structure.

Certain applications of the invention involve selection of a wavelengthappropriate for cutting a metal link without producing unacceptabledamage to a silicon substrate, where the wavelength is less than, ratherthan greater than, the conventional wavelengths of 1.047 μm and 1.064μm. This method of wavelength selection is advantageous because the useof shorter wavelengths can result in smaller laser spots, other thingsbeing equal, and hence greater ease in hitting only the desired linkwith the laser spot. In particular, other things being equal, laser spotsize is directly proportional to wavelength according to the formula:spot size is proportional to λf, where λ is the laser wavelength and fis the f-number of the optical system.

Moreover, certain applications of the invention involve selection of awavelength at which a substrate has low absorption but an interconnectmaterial has higher absorption than at conventional wavelengths of 1.047μm and 1.064 μm or higher-than-conventional wavelengths. Because of thereduced reflectivity of the interconnect material, the incident laserenergy can be reduced while the interconnect material neverthelessabsorbs sufficient energy for the interconnect to be blown withoutmultiple laser pulses (which can impact throughput) or substantialcollateral damage due to the laser beam.

The invention can effect high

quality laser link cuts on high-conductivity interconnect materials suchas copper, gold, and the like, arranged in closely-spaced patterns, withonly a single laser pulse, and without damaging the substrate. Theinvention can further allow a smaller laser spot size than would beobtainable at wavelengths of 1.047 μm, 1.064 μm, or higher, while stillproviding acceptable link cuts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a laser system according to the inventionfor removing a link of a semiconductor device, where the link ismanufactured of a material such as copper or gold.

FIG. 2 is a perspective diagrammatic view of a link on a substrate of asemiconductor device.

FIG. 3 is plot of absorption of copper, gold, aluminum, and silicon as afunction of wavelength.

FIG. 4 is a plot of a substrate absorption function according to theinvention, for copper, gold, and aluminum links on a silicon substrate,as a function of wavelength.

FIG. 5 is a plot of the function L−S for copper, gold, and aluminumlinks on a silicon substrate, where L is the absorption in the link andS is the absorption in the substrate.

FIG. 6 is a plot of the function (L−S)/(L+S) for copper, gold, andaluminum links on a silicon substrate, where L is the absorption in thelink and S is the absorption in the substrate.

DETAILED DESCRIPTION

In the block diagram of FIG. 1, a system for removing a link of asemiconductor device is shown. Laser 10 is constructed to operate at aconventional wavelength such as 1.047 μm. It is aligned to a laseroutput system that includes a wavelength shifter 12, such as a frequencydoubler or an optical parametric oscillator (OPO), constructed to shiftto a wavelength less than 0.55 μm, in the “green” region of thewavelength spectrum. As is explained in more detail below, the beam isthen passed through the remainder of the laser output system, includinga controlled electro

acousto

optic attenuator 13, a telescope 14 that expands the beam, and, ascanning head, that scans the beam over a focusing lens 16 by means oftwo scanner galvanometers, 18 and 20. The spot is focused onto wafer 22for removing links 24, under control of computer 33.

The laser 10 is mounted on a stable platform 11 relative to thegalvanometers and the work piece. It is controlled from outside of thelaser itself by computer 33 to transmit its beam to the scanner headcomprising the accurate X and Y galvanometers 18 and 20. It is veryimportant, in removing links that the beam be positioned with accuracyof less than {fraction (3/10)} of a micron. The timing of the laserpulse to correlate with the position of the continually movinggalvanometers is important. The system computer 33 asks for a laserpulse on demand.

A step and repeat table moves the wafer into position to treat eachsemiconductor device.

In one embodiment, the laser 10 is a neodymium vanadate laser, with anoverall length L of about 6 inches, and a short cavity length.

The shifter 12 of this preferred embodiment is external to the cavity,and is about another 4 inches long. In alternative embodiments, laser 10can be configured to produce a laser output having an appropriatewavelength, so that no shifter would be required.

The laser is a Q-switched diode pumped laser, of sufficient length andconstruction to enable external control of pulse rate with high accuracyby computer 33.

The cavity of the laser includes a partially transmissive mirror 7,optimized at the wavelength at which the lasing rod 6 of neodymiumvanadate is pumped by the diode. The partially transmissive outputmirror 9 is also optimized at this wavelength.

The pumping diode 4 produces between about one and two watts dependingon the design. It focuses onto the rear of the laser rod 6. Asmentioned, the laser rod is coated, on its pumped end, with a mirror 7appropriate for the standard laser wavelength of 1.064 μm or 1.047 μm.The other end of the rod is coated with a dichroic coating. Within thelaser cavity is an optical Q

switch 8 in the form of an acousto-optic modulator. It is used as theshutter for establishing the operating frequency of the laser. Beyondthe Q-switch is the output mirror 9. The two mirrors, 7 on the pumpedend of the laser rod and 9 beyond the acoustic optical Q-switch,comprise the laser cavity.

A system optical switch 13 in the form of a further acousto-opticattenuator is positioned beyond the laser cavity, in the laser outputbeam. Under control of computer 33, it serves both to prevent the beamfrom reaching the galvanometers except when desired, and, when the beamis desired at the galvanometers, to controllably reduce the power of thelaser beam to the desired power level. During vaporization proceduresthis power level may be as little as 10 percent of the gross laseroutput, depending upon operating parameters of the system and process.The power level may be about 0.1 percent of the gross laser outputduring alignment procedures in which the laser output beam is alignedwith the target structure prior to a vaporization procedure.

In operation, the positions of the X, Y galvanometers 10 and 12 arecontrolled by the computer 33 by galvanometer control G. Typically thegalvanometers move at constant speed over the semiconductor device onthe silicon wafer. The laser is controlled by timing signals based onthe timing signals that control the galvanometers. The laser operates ata constant repetition rate and is synchronized to the

galvanometers by the system optical switch 13.

In the system block diagram of FIG. 1 the laser beam is shown focusedupon the wafer. In the magnified view of FIG. 2, the laser beam is seenbeing focused on a link element 25 of a semiconductor device.

The metal link is supported on the silicon substrate 30 by silicondioxide insulator layer 32, which may be, e.g., 0.3-0.5 microns thick.Over the link is another layer of silicon dioxide (not shown). In thelink blowing technique the laser beam impinges on the link and heats itto the melting point. During the heating the metal is prevented fromvaporizing by the confining effect of the overlying layer of oxide.During the duration of the short pulse, the laser beam progressivelyheats the metal, until the metal so expands that the insulator materialruptures. At this point, the molten material is under such high pressurethat it instantly vaporizes and blows cleanly out through the rupturehole.

The wavelength produced by wavelength shifter 12 is arrived at byconsidering on an equal footing the values of both the interconnect orlink to be processed and the substrate, in such a way as to trade

off energy deposition in the substrate, which is undesirable, againstenergy deposition in the link structure, which is necessary to sever thelink. Thus, the criteria for selecting the wavelength do not require thesubstrate to be very transparent, which is especially important if thewavelength regime in which the substrate is very transparent is muchless than optimal for energy deposition in the link structure.

The criteria for selection of the appropriate wavelength are as follows:

1) Calculate the relative incident laser beam energy required to depositunit energy in the link structure. This relative incident laser beamenergy is proportional to the inverse of the absorption of the linkstructure. For example, if the link structure has an absorption of0.333, it will require three times as much incident laser energy todeposit as much energy in the link structure as it would if thestructure had an absorption of 1. FIG. 3 illustrates absorption ofcopper, gold, aluminum, and silicon as a function of wavelength (copper,gold, and aluminum being possible link structure materials and siliconbeing a substrate material).

2) Using the incident beam energy computed in step (1), calculate theenergy deposited in the substrate. For a well-matched laser spot, thisenergy will be proportional to the incident energy calculated in step(1), less the energy absorbed by the link structure, multiplied by theabsorption of the substrate. In other words, the energy absorbed in thesubstrate is proportional to (1/L−1)×S (herein, “the substrateabsorption function”), where L is the absorption in the link and S isthe absorption in the substrate.

3) Look for low values of the substrate absorption function defined instep (2) as a function of laser wavelength.

FIG. 4 illustrates the substrate absorption function for copper, gold,and aluminum links on a silicon substrate, as a function of wavelengthin the range of 0.3 to 1.4 μm. The values of the substrate absorptionfunction can be derived from the absorption curves illustrated in FIG.3, using a proportionality constant (see step (2) above) arbitrarilychosen as 0.5 for the sake of specificity (this constant merely changesthe vertical scale of FIG. 4, and does not alter any conclusions drawnfrom it).

It can be seen from FIG. 4 that for structures of gold and copper (butnot for aluminum) there is a region of wavelength less than roughly 0.55μm in which the substrate absorption function is comparable to that inthe region of wavelength greater than 1.2 μm.

It will also be noted that this function is quite different than theones presented in FIGS. 5 and 6, which illustrate two possible functionsrepresenting simple absorption contrast. More specifically, FIG. 5illustrates the function L−S, expressed as percentage, and FIG. 6illustrates the function (L−S)/(L+S). In either case, the less-than-0.55μm wavelength region is not found desirable according to FIGS. 5 and 6,even for gold or copper link structures, because the function shown inthese figures is less than zero in this region. This negative valuereflects the fact that the substrate is more absorptive than the linkstructure in this wavelength regime, and so, according to these models,this wavelength regime should not be selected.

What is claimed is:
 1. A method of vaporizing a target structure on asubstrate, comprising the steps of: calculating, as a function ofwavelength, an incident beam energy necessary to deposit unit energy inthe target structure sufficient to vaporize the target structure;calculating, for the incident beam energy, energy expected to bedeposited in the substrate as a function of wavelength; identifying awavelength below an absorption edge of the substrate, the wavelengthcorresponding to a relatively low value of the energy expected to bedeposited in the substrate, the low value being substantially less thana value of the energy expected to be deposited in the substrate at ahigher wavelength below the absorption edge of the substrate; providinga laser system configured to produce a laser output at the wavelengthcorresponding to the relatively low value of the energy expected to bedeposited in the substrate; and directing the laser output at the targetstructure on the substrate at the wavelength corresponding to therelatively low value of the energy expected to be deposited in thesubstrate and at the incident beam energy, in order to vaporize thetarget structure, the substrate being positioned beneath the targetstructure with respect to the laser output.
 2. The method of claim 1wherein the wavelength corresponding to the relatively low value of theenergy expected to be deposited in the substrate is substantially lessthan 1.047 μm.
 3. The method of claim 2 wherein the wavelengthcorresponding to the relatively low value of the energy expected to bedeposited in the substrate is less than 0.55 μm.
 4. The method of claim3 wherein the target structure comprises a metal having a conductivitygreater than that of aluminum.
 5. The method of claim 4 wherein themetal comprises copper.
 6. The method of claim 4 wherein the metalcomprises gold.
 7. The method of claim 4 wherein the substrate comprisessilicon.
 8. The method of claim 1 wherein the target structure on thesubstrate comprises a link of a semiconductor device.
 9. The method ofclaim 8 wherein the semiconductor device comprises an integratedcircuit.
 10. The method of claim 8 wherein the semiconductor devicecomprises a memory device.
 11. The method of claim 1 wherein the energyexpected to be deposited in the substrate is substantially proportionalto the incident beam energy necessary to deposit unit energy in thetarget structure minus the energy deposited in the target structure,multiplied by absorption of the substrate.
 12. A system for vaporizing atarget structure on a substrate, comprising: a laser pumping source; alaser resonator cavity configured to be pumped by the laser pumpingsource; and a laser output system configured to produce a laser outputfrom energy stored in the laser resonator cavity and to direct the laseroutput at the target structure on the substrate in order to vaporize thetarget structure, at a wavelength below an absorption edge of thesubstrate, the wavelength corresponding to a relatively low value ofenergy expected to be deposited in the substrate, the substrate beingpositioned beneath the target structure with respect to the laseroutput, given an incident beam energy necessary to deposit unit energyin the target structure sufficient to vaporize the target structure, thelow value being substantially less than a value of the energy expectedto be deposited in the substrate at a higher wavelength below theabsorption edge of the substrate, the laser output system beingconfigured to produce the laser output at the incident beam energy. 13.The system of claim 12 wherein the laser output system comprises awavelength shifter.
 14. The system of claim 12 wherein the laserresonator cavity produces laser radiation at the wavelengthcorresponding to the relatively low value of energy expected to bedeposited in the substrate.
 15. The system of claim 12 wherein thewavelength corresponding to the relatively low value of the energyexpected to be deposited in the substrate is substantially less than1.047 μm.
 16. The system of claim 15 wherein the wavelengthcorresponding to the relatively low value of the energy expected to bedeposited in the substrate is less than 0.55 μm.
 17. The system of claim16 wherein the target structure comprises a metal having a conductivitygreater than that of aluminum.
 18. The system of claim 17 wherein themetal comprises copper.
 19. The system of claim 17 wherein the metalcomprises gold.
 20. The system of claim 17 wherein the substratecomprises silicon.
 21. The system of claim 14 wherein the targetstructure on the substrate comprises a link of a semiconductor device.22. The method of claim 21 wherein the semiconductor device comprises anintegrated circuit.
 23. The method of claim 21 wherein the semiconductordevice comprises a memory device.
 24. The method of claim 14 wherein theenergy expected to be deposited in the substrate is substantiallyproportional to the incident beam energy necessary to deposit unitenergy in the target structure minus the energy deposited in the targetstructure, multiplied by absorption of the substrate.
 25. A method ofvaporizing a target structure on a substrate, comprising the steps of:providing a laser system configured to produce a laser output at thewavelength below an absorption edge of the substrate, the wavelengthcorresponding to a relatively low value of energy expected to bedeposited in the substrate, given an incident beam energy necessary todeposit unit energy in the target structure sufficient to vaporize thetarget structure, the low value being substantially less than a value ofthe energy expected to be deposited in the substrate at a higherwavelength below the absorption edge of the substrate; and directing thelaser output at the target structure on the substrate at the wavelengthcorresponding to the relatively low value of the energy expected to bedeposited in the substrate, and at the incident beam energy, in order tovaporize the target structure, the substrate being positioned beneaththe target structure with respect to the laser output.
 26. The method ofclaim 1 wherein the identified wavelength corresponding to a relativelylow value of the energy expected to be deposited in the substrate iswithin a visible region of spectrum.
 27. The method of claim 26 whereinthe identified wavelength corresponding to a relatively low value of theenergy expected to be deposited in the substrate is within a greenregion of spectrum.
 28. The method of claim 1 wherein the incident beamenergy at which the target structure is vaporized is reduced relative toan incident beam energy necessary to deposit unit energy in the targetstructure sufficient to vaporize the target structure at the higherwavelength.
 29. The system of claim 12 wherein the identified wavelengthcorresponding to a relatively low value of the energy expected to bedeposited in the substrate is within a visible region of spectrum. 30.The system of claim 29 wherein the identified wavelength correspondingto a relatively low value of the energy expected to be deposited in thesubstrate is within a green region of spectrum.
 31. The system of claim12 wherein the incident beam energy at which the target structure isvaporized is reduced relative to an incident beam energy necessary todeposit unit energy in the target structure sufficient to vaporize thetarget structure at the higher wavelength.
 32. The method of claim 25wherein the identified wavelength corresponding to a relatively lowvalue of the energy expected to be deposited in the substrate is withina visible region of spectrum.
 33. The method of claim 25 wherein theidentified wavelength corresponding to a relatively low value of theenergy expected to be deposited in the substrate is within a greenregion of spectrum.
 34. The method of claim 25 wherein the incident beamenergy at which the target structure is vaporized is reduced relative toan incident beam energy necessary to deposit unit energy in the targetstructure sufficient to vaporize the target structure at the higherwavelength.